Generally, in optical transmission systems, there may be employed an optical modulator that performs optical modulation using, for example, a Mach-Zehnder interferometer formed on a substrate (an optical modulator chip). In optical modulators using a Mach-Zehnder interferometer, an optical beam input to an optical modulator is split into two light, and an electric signal is superimposed on each of the light. These two light with the electric signal superimposed thereon are modulated into one light within an optical modulator chip, and the modulated light is output to the outside. As the optical modulator chip, for example, a lithium niobate (LiNbO3) substrate is used.
In recent years, a polarization multiplexing modulation method may be used for the purpose of speeding up the optical signal transmission. In the case of this modulation method, one light input is split into two light within an optical modulator chip, and modulation is performed on each of light. That is, two optical modulating units are formed on one optical modulator chip (one substrate). The two modulated light are output from the optical modulator chip to the outside, and after that, they are combined into one optical beam, and the combined optical beam is output to an optical fiber placed along a direction of the long side of the optical modulator chip.
A polarized-light combiner including a polarized light rotating element and a polarized light combining element may be used to combine two optical beams. The polarized-light combiner rotates the polarization direction of one of the two optical beams running in parallel by means of the polarized light rotating element such as a half-wave plate, and synthesizes the two optical beams of which the polarization directions are perpendicular to each other into one optical beam by means of the polarized light combining element such as a polarization beam combiner (PBC) prism.
[Patent Literature 1] Japanese Laid-open Patent Publication No. 63-183402
In a case where the above-described polarized-light combiner is applied to an optical modulator, the polarized-light combiner combines two optical beams output from an optical modulator chip into one optical beam, and outputs the obtained one optical beam to an optical fiber placed along the direction of the long side of a substrate. This demands a space for the placement of the polarized-light combiner and the optical fiber along the direction of the long side of the substrate, thereby it becomes difficult to achieve sufficient miniaturization of an optical modulator.
In contrast, there is considered a structure in which an optical fiber is placed along a direction of the short side of a substrate. In this structure, for example, as illustrated in FIG. 7, a mirror 700 is placed between an optical modulator chip 500 and a polarized-light combiner 600; two optical beams output from the optical modulator chip 500 are reflected in the direction of the short side of the substrate by the mirror 700 and then input to the polarized-light combiner 600. Then, the polarized-light combiner 600 synthesizes the two optical beams reflected in the direction of the short side of the substrate by the mirror 700 into one optical beam, and outputs the obtained one optical beam to the optical fiber placed along the direction of the short side of the substrate.
However, when a mirror, which is a separate part from a polarized-light combiner, is placed between an optical modulator chip and the polarized-light combiner, the number of parts increases, and the miniaturization of an optical modulator is inhibited. If the miniaturization of an optical modulator is inhibited, it becomes difficult to house the optical modulator in another device, such as an optical transponder; therefore, there are demands for promoting the miniaturization of an optical modulator.